text: /*-------------------------------------------------------------------------
   *
   * xlogreader.c
   *		Generic XLog reading facility
   *
   * Portions Copyright (c) 2013-2022, PostgreSQL Global Development Group
   *
   * IDENTIFICATION
   *		src/backend/access/transam/xlogreader.c
   *
   * NOTES
   *		See xlogreader.h for more notes on this facility.
   *
   *		This file is compiled as both front-end and backend code, so it
   *		may not use ereport, server-defined static variables, etc.
   *-------------------------------------------------------------------------
   */
  #include "postgres.h"
  
  #include <unistd.h>
  #ifdef USE_LZ4
  #include <lz4.h>
  #endif
  #ifdef USE_ZSTD
  #include <zstd.h>
  #endif
  
  #include "access/transam.h"
  #include "access/xlog_internal.h"
  #include "access/xlogreader.h"
  #include "access/xlogrecord.h"
  #include "catalog/pg_control.h"
  #include "common/pg_lzcompress.h"
  #include "replication/origin.h"
  
  #ifndef FRONTEND
  #include "miscadmin.h"
  #include "pgstat.h"
  #include "utils/memutils.h"
  #endif
  
  static void report_invalid_record(XLogReaderState *state, const char *fmt,...)
  			pg_attribute_printf(2, 3);
  static bool allocate_recordbuf(XLogReaderState *state, uint32 reclength);
  static int	ReadPageInternal(XLogReaderState *state, XLogRecPtr pageptr,
  							 int reqLen);
  static void XLogReaderInvalReadState(XLogReaderState *state);
  static bool ValidXLogRecordHeader(XLogReaderState *state, XLogRecPtr RecPtr,
  								  XLogRecPtr PrevRecPtr, XLogRecord *record, bool randAccess);
  static bool ValidXLogRecord(XLogReaderState *state, XLogRecord *record,
  							XLogRecPtr recptr);
  static void ResetDecoder(XLogReaderState *state);
  static void WALOpenSegmentInit(WALOpenSegment *seg, WALSegmentContext *segcxt,
  							   int segsize, const char *waldir);
  
  /* size of the buffer allocated for error message. */
  #define MAX_ERRORMSG_LEN 1000
  
  /*
   * Construct a string in state->errormsg_buf explaining what's wrong with
   * the current record being read.
   */
  static void
  report_invalid_record(XLogReaderState *state, const char *fmt,...)
  {
  	va_list		args;
  
  	fmt = _(fmt);
  
  	va_start(args, fmt);
  	vsnprintf(state->errormsg_buf, MAX_ERRORMSG_LEN, fmt, args);
  	va_end(args);
  }
  
  /*
   * Allocate and initialize a new XLogReader.
   *
   * Returns NULL if the xlogreader couldn't be allocated.
   */
  XLogReaderState *
  XLogReaderAllocate(int wal_segment_size, const char *waldir,
  				   XLogReaderRoutine *routine, void *private_data)
  {
  	XLogReaderState *state;
  
  	state = (XLogReaderState *)
  		palloc_extended(sizeof(XLogReaderState),
  						MCXT_ALLOC_NO_OOM | MCXT_ALLOC_ZERO);
  	if (!state)
  		return NULL;
  
  	/* initialize caller-provided support functions */
  	state->routine = *routine;
  
  	state->max_block_id = -1;
  
  	/*
  	 * Permanently allocate readBuf.  We do it this way, rather than just
  	 * making a static array, for two reasons: (1) no need to waste the
  	 * storage in most instantiations of the backend; (2) a static char array
  	 * isn't guaranteed to have any particular alignment, whereas
  	 * palloc_extended() will provide MAXALIGN'd storage.
  	 */
  	state->readBuf = (char *) palloc_extended(XLOG_BLCKSZ,
  											  MCXT_ALLOC_NO_OOM);
  	if (!state->readBuf)
  	{
  		pfree(state);
  		return NULL;
  	}
  
  	/* Initialize segment info. */
  	WALOpenSegmentInit(&state->seg, &state->segcxt, wal_segment_size,
  					   waldir);
  
  	/* system_identifier initialized to zeroes above */
  	state->private_data = private_data;
  	/* ReadRecPtr, EndRecPtr and readLen initialized to zeroes above */
  	state->errormsg_buf = palloc_extended(MAX_ERRORMSG_LEN + 1,
  										  MCXT_ALLOC_NO_OOM);
  	if (!state->errormsg_buf)
  	{
  		pfree(state->readBuf);
  		pfree(state);
  		return NULL;
  	}
  	state->errormsg_buf[0] = '\0';
  
  	/*
  	 * Allocate an initial readRecordBuf of minimal size, which can later be
  	 * enlarged if necessary.
  	 */
  	if (!allocate_recordbuf(state, 0))
  	{
  		pfree(state->errormsg_buf);
  		pfree(state->readBuf);
  		pfree(state);
  		return NULL;
  	}
  
  	return state;
  }
  
  void
  XLogReaderFree(XLogReaderState *state)
  {
  	int			block_id;
  
  	if (state->seg.ws_file != -1)
  		state->routine.segment_close(state);
  
  	for (block_id = 0; block_id <= XLR_MAX_BLOCK_ID; block_id++)
  	{
  		if (state->blocks[block_id].data)
  			pfree(state->blocks[block_id].data);
  	}
  	if (state->main_data)
  		pfree(state->main_data);
  
  	pfree(state->errormsg_buf);
  	if (state->readRecordBuf)
  		pfree(state->readRecordBuf);
  	pfree(state->readBuf);
  	pfree(state);
  }
  
  /*
   * Allocate readRecordBuf to fit a record of at least the given length.
   * Returns true if successful, false if out of memory.
   *
   * readRecordBufSize is set to the new buffer size.
   *
   * To avoid useless small increases, round its size to a multiple of
   * XLOG_BLCKSZ, and make sure it's at least 5*Max(BLCKSZ, XLOG_BLCKSZ) to start
   * with.  (That is enough for all "normal" records, but very large commit or
   * abort records might need more space.)
   */
  static bool
  allocate_recordbuf(XLogReaderState *state, uint32 reclength)
  {
  	uint32		newSize = reclength;
  
  	newSize += XLOG_BLCKSZ - (newSize % XLOG_BLCKSZ);
  	newSize = Max(newSize, 5 * Max(BLCKSZ, XLOG_BLCKSZ));
  
  #ifndef FRONTEND
  
  	/*
  	 * Note that in much unlucky circumstances, the random data read from a
  	 * recycled segment can cause this routine to be called with a size
  	 * causing a hard failure at allocation.  For a standby, this would cause
  	 * the instance to stop suddenly with a hard failure, preventing it to
  	 * retry fetching WAL from one of its sources which could allow it to move
  	 * on with replay without a manual restart. If the data comes from a past
  	 * recycled segment and is still valid, then the allocation may succeed
  	 * but record checks are going to fail so this would be short-lived.  If
  	 * the allocation fails because of a memory shortage, then this is not a
  	 * hard failure either per the guarantee given by MCXT_ALLOC_NO_OOM.
  	 */
  	if (!AllocSizeIsValid(newSize))
  		return false;
  
  #endif
  
  	if (state->readRecordBuf)
  		pfree(state->readRecordBuf);
  	state->readRecordBuf =
  		(char *) palloc_extended(newSize, MCXT_ALLOC_NO_OOM);
  	if (state->readRecordBuf == NULL)
  	{
  		state->readRecordBufSize = 0;
  		return false;
  	}
  	state->readRecordBufSize = newSize;
  	return true;
  }
  
  /*
   * Initialize the passed segment structs.
   */
  static void
  WALOpenSegmentInit(WALOpenSegment *seg, WALSegmentContext *segcxt,
  				   int segsize, const char *waldir)
  {
  	seg->ws_file = -1;
  	seg->ws_segno = 0;
  	seg->ws_tli = 0;
  
  	segcxt->ws_segsize = segsize;
  	if (waldir)
  		snprintf(segcxt->ws_dir, MAXPGPATH, "%s", waldir);
  }
  
  /*
   * Begin reading WAL at 'RecPtr'.
   *
   * 'RecPtr' should point to the beginning of a valid WAL record.  Pointing at
   * the beginning of a page is also OK, if there is a new record right after
   * the page header, i.e. not a continuation.
   *
   * This does not make any attempt to read the WAL yet, and hence cannot fail.
   * If the starting address is not correct, the first call to XLogReadRecord()
   * will error out.
   */
  void
  XLogBeginRead(XLogReaderState *state, XLogRecPtr RecPtr)
  {
  	Assert(!XLogRecPtrIsInvalid(RecPtr));
  
  	ResetDecoder(state);
  
  	/* Begin at the passed-in record pointer. */
  	state->EndRecPtr = RecPtr;
  	state->ReadRecPtr = InvalidXLogRecPtr;
  }
  
  /*
   * Attempt to read an XLOG record.
   *
   * XLogBeginRead() or XLogFindNextRecord() must be called before the first call
   * to XLogReadRecord().
   *
   * If the page_read callback fails to read the requested data, NULL is
   * returned.  The callback is expected to have reported the error; errormsg
   * is set to NULL.
   *
   * If the reading fails for some other reason, NULL is also returned, and
   * *errormsg is set to a string with details of the failure.
   *
   * The returned pointer (or *errormsg) points to an internal buffer that's
   * valid until the next call to XLogReadRecord.
   */
  XLogRecord *
  XLogReadRecord(XLogReaderState *state, char **errormsg)
  {
  	XLogRecPtr	RecPtr;
  	XLogRecord *record;
  	XLogRecPtr	targetPagePtr;
  	bool		randAccess;
  	uint32		len,
  				total_len;
  	uint32		targetRecOff;
  	uint32		pageHeaderSize;
  	bool		assembled;
  	bool		gotheader;
  	int			readOff;
  
  	/*
  	 * randAccess indicates whether to verify the previous-record pointer of
  	 * the record we're reading.  We only do this if we're reading
  	 * sequentially, which is what we initially assume.
  	 */
  	randAccess = false;
  
  	/* reset error state */
  	*errormsg = NULL;
  	state->errormsg_buf[0] = '\0';
  
  	ResetDecoder(state);
  	state->abortedRecPtr = InvalidXLogRecPtr;
  	state->missingContrecPtr = InvalidXLogRecPtr;
  
  	RecPtr = state->EndRecPtr;
  
  	if (state->ReadRecPtr != InvalidXLogRecPtr)
  	{
  		/* read the record after the one we just read */
  
  		/*
  		 * EndRecPtr is pointing to end+1 of the previous WAL record.  If
  		 * we're at a page boundary, no more records can fit on the current
  		 * page. We must skip over the page header, but we can't do that until
  		 * we've read in the page, since the header size is variable.
  		 */
  	}
  	else
  	{
  		/*
  		 * Caller supplied a position to start at.
  		 *
  		 * In this case, EndRecPtr should already be pointing to a valid
  		 * record starting position.
  		 */
  		Assert(XRecOffIsValid(RecPtr));
  		randAccess = true;
  	}
  
  restart:
  	state->currRecPtr = RecPtr;
  	assembled = false;
  
  	targetPagePtr = RecPtr - (RecPtr % XLOG_BLCKSZ);
  	targetRecOff = RecPtr % XLOG_BLCKSZ;
  
  	/*
  	 * Read the page containing the record into state->readBuf. Request enough
  	 * byte to cover the whole record header, or at least the part of it that
  	 * fits on the same page.
  	 */
  	readOff = ReadPageInternal(state, targetPagePtr,
  							   Min(targetRecOff + SizeOfXLogRecord, XLOG_BLCKSZ));
  	if (readOff < 0)
  		goto err;
  
  	/*
  	 * ReadPageInternal always returns at least the page header, so we can
  	 * examine it now.
  	 */
  	pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) state->readBuf);
  	if (targetRecOff == 0)
  	{
  		/*
  		 * At page start, so skip over page header.
  		 */
  		RecPtr += pageHeaderSize;
  		targetRecOff = pageHeaderSize;
  	}
  	else if (targetRecOff < pageHeaderSize)
  	{
  		report_invalid_record(state, "invalid record offset at %X/%X",
  							  LSN_FORMAT_ARGS(RecPtr));
  		goto err;
  	}
  
  	if ((((XLogPageHeader) state->readBuf)->xlp_info & XLP_FIRST_IS_CONTRECORD) &&
  		targetRecOff == pageHeaderSize)
  	{
  		report_invalid_record(state, "contrecord is requested by %X/%X",
  							  LSN_FORMAT_ARGS(RecPtr));
  		goto err;
  	}
  
  	/* ReadPageInternal has verified the page header */
  	Assert(pageHeaderSize <= readOff);
  
  	/*
  	 * Read the record length.
  	 *
  	 * NB: Even though we use an XLogRecord pointer here, the whole record
  	 * header might not fit on this page. xl_tot_len is the first field of the
  	 * struct, so it must be on this page (the records are MAXALIGNed), but we
  	 * cannot access any other fields until we've verified that we got the
  	 * whole header.
  	 */
  	record = (XLogRecord *) (state->readBuf + RecPtr % XLOG_BLCKSZ);
  	total_len = record->xl_tot_len;
  
  	/*
  	 * If the whole record header is on this page, validate it immediately.
  	 * Otherwise do just a basic sanity check on xl_tot_len, and validate the
  	 * rest of the header after reading it from the next page.  The xl_tot_len
  	 * check is necessary here to ensure that we enter the "Need to reassemble
  	 * record" code path below; otherwise we might fail to apply
  	 * ValidXLogRecordHeader at all.
  	 */
  	if (targetRecOff <= XLOG_BLCKSZ - SizeOfXLogRecord)
  	{
  		if (!ValidXLogRecordHeader(state, RecPtr, state->ReadRecPtr, record,
  								   randAccess))
  			goto err;
  		gotheader = true;
  	}
  	else
  	{
  		/* XXX: more validation should be done here */
  		if (total_len < SizeOfXLogRecord)
  		{
  			report_invalid_record(state,
  								  "invalid record length at %X/%X: wanted %u, got %u",
  								  LSN_FORMAT_ARGS(RecPtr),
  								  (uint32) SizeOfXLogRecord, total_len);
  			goto err;
  		}
  		gotheader = false;
  	}
  
  	len = XLOG_BLCKSZ - RecPtr % XLOG_BLCKSZ;
  	if (total_len > len)
  	{
  		/* Need to reassemble record */
  		char	   *contdata;
  		XLogPageHeader pageHeader;
  		char	   *buffer;
  		uint32		gotlen;
  
  		assembled = true;
  
  		/*
  		 * Enlarge readRecordBuf as needed.
  		 */
  		if (total_len > state->readRecordBufSize &&
  			!allocate_recordbuf(state, total_len))
  		{
  			/* We treat this as a "bogus data" condition */
  			report_invalid_record(state, "record length %u at %X/%X too long",
  								  total_len, LSN_FORMAT_ARGS(RecPtr));
  			goto err;
  		}
  
  		/* Copy the first fragment of the record from the first page. */
  		memcpy(state->readRecordBuf,
  			   state->readBuf + RecPtr % XLOG_BLCKSZ, len);
  		buffer = state->readRecordBuf + len;
  		gotlen = len;
  
  		do
  		{
  			/* Calculate pointer to beginning of next page */
  			targetPagePtr += XLOG_BLCKSZ;
  
  			/* Wait for the next page to become available */
  			readOff = ReadPageInternal(state, targetPagePtr,
  									   Min(total_len - gotlen + SizeOfXLogShortPHD,
  										   XLOG_BLCKSZ));
  
  			if (readOff < 0)
  				goto err;
  
  			Assert(SizeOfXLogShortPHD <= readOff);
  
  			pageHeader = (XLogPageHeader) state->readBuf;
  
  			/*
  			 * If we were expecting a continuation record and got an
  			 * "overwrite contrecord" flag, that means the continuation record
  			 * was overwritten with a different record.  Restart the read by
  			 * assuming the address to read is the location where we found
  			 * this flag; but keep track of the LSN of the record we were
  			 * reading, for later verification.
  			 */
  			if (pageHeader->xlp_info & XLP_FIRST_IS_OVERWRITE_CONTRECORD)
  			{
  				state->overwrittenRecPtr = RecPtr;
  				ResetDecoder(state);
  				RecPtr = targetPagePtr;
  				goto restart;
  			}
  
  			/* Check that the continuation on next page looks valid */
  			if (!(pageHeader->xlp_info & XLP_FIRST_IS_CONTRECORD))
  			{
  				report_invalid_record(state,
  									  "there is no contrecord flag at %X/%X",
  									  LSN_FORMAT_ARGS(RecPtr));
  				goto err;
  			}
  
  			/*
  			 * Cross-check that xlp_rem_len agrees with how much of the record
  			 * we expect there to be left.
  			 */
  			if (pageHeader->xlp_rem_len == 0 ||
  				total_len != (pageHeader->xlp_rem_len + gotlen))
  			{
  				report_invalid_record(state,
  									  "invalid contrecord length %u (expected %lld) at %X/%X",
  									  pageHeader->xlp_rem_len,
  									  ((long long) total_len) - gotlen,
  									  LSN_FORMAT_ARGS(RecPtr));
  				goto err;
  			}
  
  			/* Append the continuation from this page to the buffer */
  			pageHeaderSize = XLogPageHeaderSize(pageHeader);
  
  			if (readOff < pageHeaderSize)
  				readOff = ReadPageInternal(state, targetPagePtr,
  										   pageHeaderSize);
  
  			Assert(pageHeaderSize <= readOff);
  
  			contdata = (char *) state->readBuf + pageHeaderSize;
  			len = XLOG_BLCKSZ - pageHeaderSize;
  			if (pageHeader->xlp_rem_len < len)
  				len = pageHeader->xlp_rem_len;
  
  			if (readOff < pageHeaderSize + len)
  				readOff = ReadPageInternal(state, targetPagePtr,
  										   pageHeaderSize + len);
  
  			memcpy(buffer, (char *) contdata, len);
  			buffer += len;
  			gotlen += len;
  
  			/* If we just reassembled the record header, validate it. */
  			if (!gotheader)
  			{
  				record = (XLogRecord *) state->readRecordBuf;
  				if (!ValidXLogRecordHeader(state, RecPtr, state->ReadRecPtr,
  										   record, randAccess))
  					goto err;
  				gotheader = true;
  			}
  		} while (gotlen < total_len);
  
  		Assert(gotheader);
  
  		record = (XLogRecord *) state->readRecordBuf;
  		if (!ValidXLogRecord(state, record, RecPtr))
  			goto err;
  
  		pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) state->readBuf);
  		state->ReadRecPtr = RecPtr;
  		state->EndRecPtr = targetPagePtr + pageHeaderSize
  			+ MAXALIGN(pageHeader->xlp_rem_len);
  	}
  	else
  	{
  		/* Wait for the record data to become available */
  		readOff = ReadPageInternal(state, targetPagePtr,
  								   Min(targetRecOff + total_len, XLOG_BLCKSZ));
  		if (readOff < 0)
  			goto err;
  
  		/* Record does not cross a page boundary */
  		if (!ValidXLogRecord(state, record, RecPtr))
  			goto err;
  
  		state->EndRecPtr = RecPtr + MAXALIGN(total_len);
  
  		state->ReadRecPtr = RecPtr;
  	}
  
  	/*
  	 * Special processing if it's an XLOG SWITCH record
  	 */
  	if (record->xl_rmid == RM_XLOG_ID &&
  		(record->xl_info & ~XLR_INFO_MASK) == XLOG_SWITCH)
  	{
  		/* Pretend it extends to end of segment */
  		state->EndRecPtr += state->segcxt.ws_segsize - 1;
  		state->EndRecPtr -= XLogSegmentOffset(state->EndRecPtr, state->segcxt.ws_segsize);
  	}
  
  	if (DecodeXLogRecord(state, record, errormsg))
  		return record;
  	else
  		return NULL;
  
  err:
  	if (assembled)
  	{
  		/*
  		 * We get here when a record that spans multiple pages needs to be
  		 * assembled, but something went wrong -- perhaps a contrecord piece
  		 * was lost.  If caller is WAL replay, it will know where the aborted
  		 * record was and where to direct followup WAL to be written, marking
  		 * the next piece with XLP_FIRST_IS_OVERWRITE_CONTRECORD, which will
  		 * in turn signal downstream WAL consumers that the broken WAL record
  		 * is to be ignored.
  		 */
  		state->abortedRecPtr = RecPtr;
  		state->missingContrecPtr = targetPagePtr;
  	}
  
  	/*
  	 * Invalidate the read state. We might read from a different source after
  	 * failure.
  	 */
  	XLogReaderInvalReadState(state);
  
  	if (state->errormsg_buf[0] != '\0')
  		*errormsg = state->errormsg_buf;
  
  	return NULL;
  }
  
  /*
   * Read a single xlog page including at least [pageptr, reqLen] of valid data
   * via the page_read() callback.
   *
   * Returns -1 if the required page cannot be read for some reason; errormsg_buf
   * is set in that case (unless the error occurs in the page_read callback).
   *
   * We fetch the page from a reader-local cache if we know we have the required
   * data and if there hasn't been any error since caching the data.
   */
  static int
  ReadPageInternal(XLogReaderState *state, XLogRecPtr pageptr, int reqLen)
  {
  	int			readLen;
  	uint32		targetPageOff;
  	XLogSegNo	targetSegNo;
  	XLogPageHeader hdr;
  
  	Assert((pageptr % XLOG_BLCKSZ) == 0);
  
  	XLByteToSeg(pageptr, targetSegNo, state->segcxt.ws_segsize);
  	targetPageOff = XLogSegmentOffset(pageptr, state->segcxt.ws_segsize);
  
  	/* check whether we have all the requested data already */
  	if (targetSegNo == state->seg.ws_segno &&
  		targetPageOff == state->segoff && reqLen <= state->readLen)
  		return state->readLen;
  
  	/*
  	 * Data is not in our buffer.
  	 *
  	 * Every time we actually read the segment, even if we looked at parts of
  	 * it before, we need to do verification as the page_read callback might
  	 * now be rereading data from a different source.
  	 *
  	 * Whenever switching to a new WAL segment, we read the first page of the
  	 * file and validate its header, even if that's not where the target
  	 * record is.  This is so that we can check the additional identification
  	 * info that is present in the first page's "long" header.
  	 */
  	if (targetSegNo != state->seg.ws_segno && targetPageOff != 0)
  	{
  		XLogRecPtr	targetSegmentPtr = pageptr - targetPageOff;
  
  		readLen = state->routine.page_read(state, targetSegmentPtr, XLOG_BLCKSZ,
  										   state->currRecPtr,
  										   state->readBuf);
  		if (readLen < 0)
  			goto err;
  
  		/* we can be sure to have enough WAL available, we scrolled back */
  		Assert(readLen == XLOG_BLCKSZ);
  
  		if (!XLogReaderValidatePageHeader(state, targetSegmentPtr,
  										  state->readBuf))
  			goto err;
  	}
  
  	/*
  	 * First, read the requested data length, but at least a short page header
  	 * so that we can validate it.
  	 */
  	readLen = state->routine.page_read(state, pageptr, Max(reqLen, SizeOfXLogShortPHD),
  									   state->currRecPtr,
  									   state->readBuf);
  	if (readLen < 0)
  		goto err;
  
  	Assert(readLen <= XLOG_BLCKSZ);
  
  	/* Do we have enough data to check the header length? */
  	if (readLen <= SizeOfXLogShortPHD)
  		goto err;
  
  	Assert(readLen >= reqLen);
  
  	hdr = (XLogPageHeader) state->readBuf;
  
  	/* still not enough */
  	if (readLen < XLogPageHeaderSize(hdr))
  	{
  		readLen = state->routine.page_read(state, pageptr, XLogPageHeaderSize(hdr),
  										   state->currRecPtr,
  										   state->readBuf);
  		if (readLen < 0)
  			goto err;
  	}
  
  	/*
  	 * Now that we know we have the full header, validate it.
  	 */
  	if (!XLogReaderValidatePageHeader(state, pageptr, (char *) hdr))
  		goto err;
  
  	/* update read state information */
  	state->seg.ws_segno = targetSegNo;
  	state->segoff = targetPageOff;
  	state->readLen = readLen;
  
  	return readLen;
  
  err:
  	XLogReaderInvalReadState(state);
  	return -1;
  }
  
  /*
   * Invalidate the xlogreader's read state to force a re-read.
   */
  static void
  XLogReaderInvalReadState(XLogReaderState *state)
  {
  	state->seg.ws_segno = 0;
  	state->segoff = 0;
  	state->readLen = 0;
  }
  
  /*
   * Validate an XLOG record header.
   *
   * This is just a convenience subroutine to avoid duplicated code in
   * XLogReadRecord.  It's not intended for use from anywhere else.
   */
  static bool
  ValidXLogRecordHeader(XLogReaderState *state, XLogRecPtr RecPtr,
  					  XLogRecPtr PrevRecPtr, XLogRecord *record,
  					  bool randAccess)
  {
  	if (record->xl_tot_len < SizeOfXLogRecord)
  	{
  		report_invalid_record(state,
  							  "invalid record length at %X/%X: wanted %u, got %u",
  							  LSN_FORMAT_ARGS(RecPtr),
  							  (uint32) SizeOfXLogRecord, record->xl_tot_len);
  		return false;
  	}
  	if (record->xl_rmid > RM_MAX_ID)
  	{
  		report_invalid_record(state,
  							  "invalid resource manager ID %u at %X/%X",
  							  record->xl_rmid, LSN_FORMAT_ARGS(RecPtr));
  		return false;
  	}
  	if (randAccess)
  	{
  		/*
  		 * We can't exactly verify the prev-link, but surely it should be less
  		 * than the record's own address.
  		 */
  		if (!(record->xl_prev < RecPtr))
  		{
  			report_invalid_record(state,
  								  "record with incorrect prev-link %X/%X at %X/%X",
  								  LSN_FORMAT_ARGS(record->xl_prev),
  								  LSN_FORMAT_ARGS(RecPtr));
  			return false;
  		}
  	}
  	else
  	{
  		/*
  		 * Record's prev-link should exactly match our previous location. This
  		 * check guards against torn WAL pages where a stale but valid-looking
  		 * WAL record starts on a sector boundary.
  		 */
  		if (record->xl_prev != PrevRecPtr)
  		{
  			report_invalid_record(state,
  								  "record with incorrect prev-link %X/%X at %X/%X",
  								  LSN_FORMAT_ARGS(record->xl_prev),
  								  LSN_FORMAT_ARGS(RecPtr));
  			return false;
  		}
  	}
  
  	return true;
  }
  
  
  /*
   * CRC-check an XLOG record.  We do not believe the contents of an XLOG
   * record (other than to the minimal extent of computing the amount of
   * data to read in) until we've checked the CRCs.
   *
   * We assume all of the record (that is, xl_tot_len bytes) has been read
   * into memory at *record.  Also, ValidXLogRecordHeader() has accepted the
   * record's header, which means in particular that xl_tot_len is at least
   * SizeOfXLogRecord.
   */
  static bool
  ValidXLogRecord(XLogReaderState *state, XLogRecord *record, XLogRecPtr recptr)
  {
  	pg_crc32c	crc;
  
  	/* Calculate the CRC */
  	INIT_CRC32C(crc);
  	COMP_CRC32C(crc, ((char *) record) + SizeOfXLogRecord, record->xl_tot_len - SizeOfXLogRecord);
  	/* include the record header last */
  	COMP_CRC32C(crc, (char *) record, offsetof(XLogRecord, xl_crc));
  	FIN_CRC32C(crc);
  
  	if (!EQ_CRC32C(record->xl_crc, crc))
  	{
  		report_invalid_record(state,
  							  "incorrect resource manager data checksum in record at %X/%X",
  							  LSN_FORMAT_ARGS(recptr));
  		return false;
  	}
  
  	return true;
  }
  
  /*
   * Validate a page header.
   *
   * Check if 'phdr' is valid as the header of the XLog page at position
   * 'recptr'.
   */
  bool
  XLogReaderValidatePageHeader(XLogReaderState *state, XLogRecPtr recptr,
  							 char *phdr)
  {
  	XLogRecPtr	recaddr;
  	XLogSegNo	segno;
  	int32		offset;
  	XLogPageHeader hdr = (XLogPageHeader) phdr;
  
  	Assert((recptr % XLOG_BLCKSZ) == 0);
  
  	XLByteToSeg(recptr, segno, state->segcxt.ws_segsize);
  	offset = XLogSegmentOffset(recptr, state->segcxt.ws_segsize);
  
  	XLogSegNoOffsetToRecPtr(segno, offset, state->segcxt.ws_segsize, recaddr);
  
  	if (hdr->xlp_magic != XLOG_PAGE_MAGIC)
  	{
  		char		fname[MAXFNAMELEN];
  
  		XLogFileName(fname, state->seg.ws_tli, segno, state->segcxt.ws_segsize);
  
  		report_invalid_record(state,
  							  "invalid magic number %04X in log segment %s, offset %u",
  							  hdr->xlp_magic,
  							  fname,
  							  offset);
  		return false;
  	}
  
  	if ((hdr->xlp_info & ~XLP_ALL_FLAGS) != 0)
  	{
  		char		fname[MAXFNAMELEN];
  
  		XLogFileName(fname, state->seg.ws_tli, segno, state->segcxt.ws_segsize);
  
  		report_invalid_record(state,
  							  "invalid info bits %04X in log segment %s, offset %u",
  							  hdr->xlp_info,
  							  fname,
  							  offset);
  		return false;
  	}
  
  	if (hdr->xlp_info & XLP_LONG_HEADER)
  	{
  		XLogLongPageHeader longhdr = (XLogLongPageHeader) hdr;
  
  		if (state->system_identifier &&
  			longhdr->xlp_sysid != state->system_identifier)
  		{
  			report_invalid_record(state,
  								  "WAL file is from different database system: WAL file database system identifier is %llu, pg_control database system identifier is %llu",
  								  (unsigned long long) longhdr->xlp_sysid,
  								  (unsigned long long) state->system_identifier);
  			return false;
  		}
  		else if (longhdr->xlp_seg_size != state->segcxt.ws_segsize)
  		{
  			report_invalid_record(state,
  								  "WAL file is from different database system: incorrect segment size in page header");
  			return false;
  		}
  		else if (longhdr->xlp_xlog_blcksz != XLOG_BLCKSZ)
  		{
  			report_invalid_record(state,
  								  "WAL file is from different database system: incorrect XLOG_BLCKSZ in page header");
  			return false;
  		}
  	}
  	else if (offset == 0)
  	{
  		char		fname[MAXFNAMELEN];
  
  		XLogFileName(fname, state->seg.ws_tli, segno, state->segcxt.ws_segsize);
  
  		/* hmm, first page of file doesn't have a long header? */
  		report_invalid_record(state,
  							  "invalid info bits %04X in log segment %s, offset %u",
  							  hdr->xlp_info,
  							  fname,
  							  offset);
  		return false;
  	}
  
  	/*
  	 * Check that the address on the page agrees with what we expected. This
  	 * check typically fails when an old WAL segment is recycled, and hasn't
  	 * yet been overwritten with new data yet.
  	 */
  	if (hdr->xlp_pageaddr != recaddr)
  	{
  		char		fname[MAXFNAMELEN];
  
  		XLogFileName(fname, state->seg.ws_tli, segno, state->segcxt.ws_segsize);
  
  		report_invalid_record(state,
  							  "unexpected pageaddr %X/%X in log segment %s, offset %u",
  							  LSN_FORMAT_ARGS(hdr->xlp_pageaddr),
  							  fname,
  							  offset);
  		return false;
  	}
  
  	/*
  	 * Since child timelines are always assigned a TLI greater than their
  	 * immediate parent's TLI, we should never see TLI go backwards across
  	 * successive pages of a consistent WAL sequence.
  	 *
  	 * Sometimes we re-read a segment that's already been (partially) read. So
  	 * we only verify TLIs for pages that are later than the last remembered
  	 * LSN.
  	 */
  	if (recptr > state->latestPagePtr)
  	{
  		if (hdr->xlp_tli < state->latestPageTLI)
  		{
  			char		fname[MAXFNAMELEN];
  
  			XLogFileName(fname, state->seg.ws_tli, segno, state->segcxt.ws_segsize);
  
  			report_invalid_record(state,
  								  "out-of-sequence timeline ID %u (after %u) in log segment %s, offset %u",
  								  hdr->xlp_tli,
  								  state->latestPageTLI,
  								  fname,
  								  offset);
  			return false;
  		}
  	}
  	state->latestPagePtr = recptr;
  	state->latestPageTLI = hdr->xlp_tli;
  
  	return true;
  }
  
  #ifdef FRONTEND
  /*
   * Functions that are currently not needed in the backend, but are better
   * implemented inside xlogreader.c because of the internal facilities available
   * here.
   */
  
  /*
   * Find the first record with an lsn >= RecPtr.
   *
   * This is different from XLogBeginRead() in that RecPtr doesn't need to point
   * to a valid record boundary.  Useful for checking whether RecPtr is a valid
   * xlog address for reading, and to find the first valid address after some
   * address when dumping records for debugging purposes.
   *
   * This positions the reader, like XLogBeginRead(), so that the next call to
   * XLogReadRecord() will read the next valid record.
   */
  XLogRecPtr
  XLogFindNextRecord(XLogReaderState *state, XLogRecPtr RecPtr)
  {
  	XLogRecPtr	tmpRecPtr;
  	XLogRecPtr	found = InvalidXLogRecPtr;
  	XLogPageHeader header;
  	char	   *errormsg;
  
  	Assert(!XLogRecPtrIsInvalid(RecPtr));
  
  	/*
  	 * skip over potential continuation data, keeping in mind that it may span
  	 * multiple pages
  	 */
  	tmpRecPtr = RecPtr;
  	while (true)
  	{
  		XLogRecPtr	targetPagePtr;
  		int			targetRecOff;
  		uint32		pageHeaderSize;
  		int			readLen;
  
  		/*
  		 * Compute targetRecOff. It should typically be equal or greater than
  		 * short page-header since a valid record can't start anywhere before
  		 * that, except when caller has explicitly specified the offset that
  		 * falls somewhere there or when we are skipping multi-page
  		 * continuation record. It doesn't matter though because
  		 * ReadPageInternal() is prepared to handle that and will read at
  		 * least short page-header worth of data
  		 */
  		targetRecOff = tmpRecPtr % XLOG_BLCKSZ;
  
  		/* scroll back to page boundary */
  		targetPagePtr = tmpRecPtr - targetRecOff;
  
  		/* Read the page containing the record */
  		readLen = ReadPageInternal(state, targetPagePtr, targetRecOff);
  		if (readLen < 0)
  			goto err;
  
  		header = (XLogPageHeader) state->readBuf;
  
  		pageHeaderSize = XLogPageHeaderSize(header);
  
  		/* make sure we have enough data for the page header */
  		readLen = ReadPageInternal(state, targetPagePtr, pageHeaderSize);
  		if (readLen < 0)
  			goto err;
  
  		/* skip over potential continuation data */
  		if (header->xlp_info & XLP_FIRST_IS_CONTRECORD)
  		{
  			/*
  			 * If the length of the remaining continuation data is more than
  			 * what can fit in this page, the continuation record crosses over
  			 * this page. Read the next page and try again. xlp_rem_len in the
  			 * next page header will contain the remaining length of the
  			 * continuation data
  			 *
  			 * Note that record headers are MAXALIGN'ed
  			 */
  			if (MAXALIGN(header->xlp_rem_len) >= (XLOG_BLCKSZ - pageHeaderSize))
  				tmpRecPtr = targetPagePtr + XLOG_BLCKSZ;
  			else
  			{
  				/*
  				 * The previous continuation record ends in this page. Set
  				 * tmpRecPtr to point to the first valid record
  				 */
  				tmpRecPtr = targetPagePtr + pageHeaderSize
  					+ MAXALIGN(header->xlp_rem_len);
  				break;
  			}
  		}
  		else
  		{
  			tmpRecPtr = targetPagePtr + pageHeaderSize;
  			break;
  		}
  	}
  
  	/*
  	 * we know now that tmpRecPtr is an address pointing to a valid XLogRecord
  	 * because either we're at the first record after the beginning of a page
  	 * or we just jumped over the remaining data of a continuation.
  	 */
  	XLogBeginRead(state, tmpRecPtr);
  	while (XLogReadRecord(state, &errormsg) != NULL)
  	{
  		/* past the record we've found, break out */
  		if (RecPtr <= state->ReadRecPtr)
  		{
  			/* Rewind the reader to the beginning of the last record. */
  			found = state->ReadRecPtr;
  			XLogBeginRead(state, found);
  			return found;
  		}
  	}
  
  err:
  	XLogReaderInvalReadState(state);
  
  	return InvalidXLogRecPtr;
  }
  
  #endif							/* FRONTEND */
  
  /*
   * Helper function to ease writing of XLogRoutine->page_read callbacks.
   * If this function is used, caller must supply a segment_open callback in
   * 'state', as that is used here.
   *
   * Read 'count' bytes into 'buf', starting at location 'startptr', from WAL
   * fetched from timeline 'tli'.
   *
   * Returns true if succeeded, false if an error occurs, in which case
   * 'errinfo' receives error details.
   *
   * XXX probably this should be improved to suck data directly from the
   * WAL buffers when possible.
   */
  bool
  WALRead(XLogReaderState *state,
  		char *buf, XLogRecPtr startptr, Size count, TimeLineID tli,
  		WALReadError *errinfo)
  {
  	char	   *p;
  	XLogRecPtr	recptr;
  	Size		nbytes;
  
  	p = buf;
  	recptr = startptr;
  	nbytes = count;
  
  	while (nbytes > 0)
  	{
  		uint32		startoff;
  		int			segbytes;
  		int			readbytes;
  
  		startoff = XLogSegmentOffset(recptr, state->segcxt.ws_segsize);
  
  		/*
  		 * If the data we want is not in a segment we have open, close what we
  		 * have (if anything) and open the next one, using the caller's
  		 * provided openSegment callback.
  		 */
  		if (state->seg.ws_file < 0 ||
  			!XLByteInSeg(recptr, state->seg.ws_segno, state->segcxt.ws_segsize) ||
  			tli != state->seg.ws_tli)
  		{
  			XLogSegNo	nextSegNo;
  
  			if (state->seg.ws_file >= 0)
  				state->routine.segment_close(state);
  
  			XLByteToSeg(recptr, nextSegNo, state->segcxt.ws_segsize);
  			state->routine.segment_open(state, nextSegNo, &tli);
  
  			/* This shouldn't happen -- indicates a bug in segment_open */
  			Assert(state->seg.ws_file >= 0);
  
  			/* Update the current segment info. */
  			state->seg.ws_tli = tli;
  			state->seg.ws_segno = nextSegNo;
  		}
  
  		/* How many bytes are within this segment? */
  		if (nbytes > (state->segcxt.ws_segsize - startoff))
  			segbytes = state->segcxt.ws_segsize - startoff;
  		else
  			segbytes = nbytes;
  
  #ifndef FRONTEND
  		pgstat_report_wait_start(WAIT_EVENT_WAL_READ);
  #endif
  
  		/* Reset errno first; eases reporting non-errno-affecting errors */
  		errno = 0;
  		readbytes = pg_pread(state->seg.ws_file, p, segbytes, (off_t) startoff);
  
  #ifndef FRONTEND
  		pgstat_report_wait_end();
  #endif
  
  		if (readbytes <= 0)
  		{
  			errinfo->wre_errno = errno;
  			errinfo->wre_req = segbytes;
  			errinfo->wre_read = readbytes;
  			errinfo->wre_off = startoff;
  			errinfo->wre_seg = state->seg;
  			return false;
  		}
  
  		/* Update state for read */
  		recptr += readbytes;
  		nbytes -= readbytes;
  		p += readbytes;
  	}
  
  	return true;
  }
  
  /* ----------------------------------------
   * Functions for decoding the data and block references in a record.
   * ----------------------------------------
   */
  
  /* private function to reset the state between records */
  static void
  ResetDecoder(XLogReaderState *state)
  {
  	int			block_id;
  
  	state->decoded_record = NULL;
  
  	state->main_data_len = 0;
  
  	for (block_id = 0; block_id <= state->max_block_id; block_id++)
  	{
  		state->blocks[block_id].in_use = false;
  		state->blocks[block_id].has_image = false;
  		state->blocks[block_id].has_data = false;
  		state->blocks[block_id].apply_image = false;
  	}
  	state->max_block_id = -1;
  }
  
  /*
   * Decode the previously read record.
   *
   * On error, a human-readable error message is returned in *errormsg, and
   * the return value is false.
   */
  bool
  DecodeXLogRecord(XLogReaderState *state, XLogRecord *record, char **errormsg)
  {
  	/*
  	 * read next _size bytes from record buffer, but check for overrun first.
  	 */
  #define COPY_HEADER_FIELD(_dst, _size)			\
  	do {										\
  		if (remaining < _size)					\
  			goto shortdata_err;					\
  		memcpy(_dst, ptr, _size);				\
  		ptr += _size;							\
  		remaining -= _size;						\
  	} while(0)
  
  	char	   *ptr;
  	uint32		remaining;
  	uint32		datatotal;
  	RelFileNode *rnode = NULL;
  	uint8		block_id;
  
  	ResetDecoder(state);
  
  	state->decoded_record = record;
  	state->record_origin = InvalidRepOriginId;
  	state->toplevel_xid = InvalidTransactionId;
  
  	ptr = (char *) record;
  	ptr += SizeOfXLogRecord;
  	remaining = record->xl_tot_len - SizeOfXLogRecord;
  
  	/* Decode the headers */
  	datatotal = 0;
  	while (remaining > datatotal)
  	{
  		COPY_HEADER_FIELD(&block_id, sizeof(uint8));
  
  		if (block_id == XLR_BLOCK_ID_DATA_SHORT)
  		{
  			/* XLogRecordDataHeaderShort */
  			uint8		main_data_len;
  
  			COPY_HEADER_FIELD(&main_data_len, sizeof(uint8));
  
  			state->main_data_len = main_data_len;
  			datatotal += main_data_len;
  			break;				/* by convention, the main data fragment is
  								 * always last */
  		}
  		else if (block_id == XLR_BLOCK_ID_DATA_LONG)
  		{
  			/* XLogRecordDataHeaderLong */
  			uint32		main_data_len;
  
  			COPY_HEADER_FIELD(&main_data_len, sizeof(uint32));
  			state->main_data_len = main_data_len;
  			datatotal += main_data_len;
  			break;				/* by convention, the main data fragment is
  								 * always last */
  		}
  		else if (block_id == XLR_BLOCK_ID_ORIGIN)
  		{
  			COPY_HEADER_FIELD(&state->record_origin, sizeof(RepOriginId));
  		}
  		else if (block_id == XLR_BLOCK_ID_TOPLEVEL_XID)
  		{
  			COPY_HEADER_FIELD(&state->toplevel_xid, sizeof(TransactionId));
  		}
  		else if (block_id <= XLR_MAX_BLOCK_ID)
  		{
  			/* XLogRecordBlockHeader */
  			DecodedBkpBlock *blk;
  			uint8		fork_flags;
  
  			if (block_id <= state->max_block_id)
  			{
  				report_invalid_record(state,
  									  "out-of-order block_id %u at %X/%X",
  									  block_id,
  									  LSN_FORMAT_ARGS(state->ReadRecPtr));
  				goto err;
  			}
  			state->max_block_id = block_id;
  
  			blk = &state->blocks[block_id];
  			blk->in_use = true;
  			blk->apply_image = false;
  
  			COPY_HEADER_FIELD(&fork_flags, sizeof(uint8));
  			blk->forknum = fork_flags & BKPBLOCK_FORK_MASK;
  			blk->flags = fork_flags;
  			blk->has_image = ((fork_flags & BKPBLOCK_HAS_IMAGE) != 0);
  			blk->has_data = ((fork_flags & BKPBLOCK_HAS_DATA) != 0);
  
  			COPY_HEADER_FIELD(&blk->data_len, sizeof(uint16));
  			/* cross-check that the HAS_DATA flag is set iff data_length > 0 */
  			if (blk->has_data && blk->data_len == 0)
  			{
  				report_invalid_record(state,
  									  "BKPBLOCK_HAS_DATA set, but no data included at %X/%X",
  									  LSN_FORMAT_ARGS(state->ReadRecPtr));
  				goto err;
  			}
  			if (!blk->has_data && blk->data_len != 0)
  			{
  				report_invalid_record(state,
  									  "BKPBLOCK_HAS_DATA not set, but data length is %u at %X/%X",
  									  (unsigned int) blk->data_len,
  									  LSN_FORMAT_ARGS(state->ReadRecPtr));
  				goto err;
  			}
  			datatotal += blk->data_len;
  
  			if (blk->has_image)
  			{
  				COPY_HEADER_FIELD(&blk->bimg_len, sizeof(uint16));
  				COPY_HEADER_FIELD(&blk->hole_offset, sizeof(uint16));
  				COPY_HEADER_FIELD(&blk->bimg_info, sizeof(uint8));
  
  				blk->apply_image = ((blk->bimg_info & BKPIMAGE_APPLY) != 0);
  
  				if (BKPIMAGE_COMPRESSED(blk->bimg_info))
  				{
  					if (blk->bimg_info & BKPIMAGE_HAS_HOLE)
  						COPY_HEADER_FIELD(&blk->hole_length, sizeof(uint16));
  					else
  						blk->hole_length = 0;
  				}
  				else
  					blk->hole_length = BLCKSZ - blk->bimg_len;
  				datatotal += blk->bimg_len;
  
  				/*
  				 * cross-check that hole_offset > 0, hole_length > 0 and
  				 * bimg_len < BLCKSZ if the HAS_HOLE flag is set.
  				 */
  				if ((blk->bimg_info & BKPIMAGE_HAS_HOLE) &&
  					(blk->hole_offset == 0 ||
  					 blk->hole_length == 0 ||
  					 blk->bimg_len == BLCKSZ))
  				{
  					report_invalid_record(state,
  										  "BKPIMAGE_HAS_HOLE set, but hole offset %u length %u block image length %u at %X/%X",
  										  (unsigned int) blk->hole_offset,
  										  (unsigned int) blk->hole_length,
  										  (unsigned int) blk->bimg_len,
  										  LSN_FORMAT_ARGS(state->ReadRecPtr));
  					goto err;
  				}
  
  				/*
  				 * cross-check that hole_offset == 0 and hole_length == 0 if
  				 * the HAS_HOLE flag is not set.
  				 */
  				if (!(blk->bimg_info & BKPIMAGE_HAS_HOLE) &&
  					(blk->hole_offset != 0 || blk->hole_length != 0))
  				{
  					report_invalid_record(state,
  										  "BKPIMAGE_HAS_HOLE not set, but hole offset %u length %u at %X/%X",
  										  (unsigned int) blk->hole_offset,
  										  (unsigned int) blk->hole_length,
  										  LSN_FORMAT_ARGS(state->ReadRecPtr));
  					goto err;
  				}
  
  				/*
  				 * Cross-check that bimg_len < BLCKSZ if it is compressed.
  				 */
  				if (BKPIMAGE_COMPRESSED(blk->bimg_info) &&
  					blk->bimg_len == BLCKSZ)
  				{
  					report_invalid_record(state,
  										  "BKPIMAGE_COMPRESSED set, but block image length %u at %X/%X",
  										  (unsigned int) blk->bimg_len,
  										  LSN_FORMAT_ARGS(state->ReadRecPtr));
  					goto err;
  				}
  
  				/*
  				 * cross-check that bimg_len = BLCKSZ if neither HAS_HOLE is
  				 * set nor COMPRESSED().
  				 */
  				if (!(blk->bimg_info & BKPIMAGE_HAS_HOLE) &&
  					!BKPIMAGE_COMPRESSED(blk->bimg_info) &&
  					blk->bimg_len != BLCKSZ)
  				{
  					report_invalid_record(state,
  										  "neither BKPIMAGE_HAS_HOLE nor BKPIMAGE_COMPRESSED set, but block image length is %u at %X/%X",
  										  (unsigned int) blk->data_len,
  										  LSN_FORMAT_ARGS(state->ReadRecPtr));
  					goto err;
  				}
  			}
  			if (!(fork_flags & BKPBLOCK_SAME_REL))
  			{
  				COPY_HEADER_FIELD(&blk->rnode, sizeof(RelFileNode));
  				rnode = &blk->rnode;
  			}
  			else
  			{
  				if (rnode == NULL)
  				{
  					report_invalid_record(state,
  										  "BKPBLOCK_SAME_REL set but no previous rel at %X/%X",
  										  LSN_FORMAT_ARGS(state->ReadRecPtr));
  					goto err;
  				}
  
  				blk->rnode = *rnode;
  			}
  			COPY_HEADER_FIELD(&blk->blkno, sizeof(BlockNumber));
  		}
  		else
  		{
  			report_invalid_record(state,
  								  "invalid block_id %u at %X/%X",
  								  block_id, LSN_FORMAT_ARGS(state->ReadRecPtr));
  			goto err;
  		}
  	}
  
  	if (remaining != datatotal)
  		goto shortdata_err;
  
  	/*
  	 * Ok, we've parsed the fragment headers, and verified that the total
  	 * length of the payload in the fragments is equal to the amount of data
  	 * left. Copy the data of each fragment to a separate buffer.
  	 *
  	 * We could just set up pointers into readRecordBuf, but we want to align
  	 * the data for the convenience of the callers. Backup images are not
  	 * copied, however; they don't need alignment.
  	 */
  
  	/* block data first */
  	for (block_id = 0; block_id <= state->max_block_id; block_id++)
  	{
  		DecodedBkpBlock *blk = &state->blocks[block_id];
  
  		if (!blk->in_use)
  			continue;
  
  		Assert(blk->has_image || !blk->apply_image);
  
  		if (blk->has_image)
  		{
  			blk->bkp_image = ptr;
  			ptr += blk->bimg_len;
  		}
  		if (blk->has_data)
  		{
  			if (!blk->data || blk->data_len > blk->data_bufsz)
  			{
  				if (blk->data)
  					pfree(blk->data);
  
  				/*
  				 * Force the initial request to be BLCKSZ so that we don't
  				 * waste time with lots of trips through this stanza as a
  				 * result of WAL compression.
  				 */
  				blk->data_bufsz = MAXALIGN(Max(blk->data_len, BLCKSZ));
  				blk->data = palloc(blk->data_bufsz);
  			}
  			memcpy(blk->data, ptr, blk->data_len);
  			ptr += blk->data_len;
  		}
  	}
  
  	/* and finally, the main data */
  	if (state->main_data_len > 0)
  	{
  		if (!state->main_data || state->main_data_len > state->main_data_bufsz)
  		{
  			if (state->main_data)
  				pfree(state->main_data);
  
  			/*
  			 * main_data_bufsz must be MAXALIGN'ed.  In many xlog record
  			 * types, we omit trailing struct padding on-disk to save a few
  			 * bytes; but compilers may generate accesses to the xlog struct
  			 * that assume that padding bytes are present.  If the palloc
  			 * request is not large enough to include such padding bytes then
  			 * we'll get valgrind complaints due to otherwise-harmless fetches
  			 * of the padding bytes.
  			 *
  			 * In addition, force the initial request to be reasonably large
  			 * so that we don't waste time with lots of trips through this
  			 * stanza.  BLCKSZ / 2 seems like a good compromise choice.
  			 */
  			state->main_data_bufsz = MAXALIGN(Max(state->main_data_len,
  												  BLCKSZ / 2));
  			state->main_data = palloc(state->main_data_bufsz);
  		}
  		memcpy(state->main_data, ptr, state->main_data_len);
  		ptr += state->main_data_len;
  	}
  
  	return true;
  
  shortdata_err:
  	report_invalid_record(state,
  						  "record with invalid length at %X/%X",
  						  LSN_FORMAT_ARGS(state->ReadRecPtr));
  err:
  	*errormsg = state->errormsg_buf;
  
  	return false;
  }
  
  /*
   * Returns information about the block that a block reference refers to.
   *
   * If the WAL record contains a block reference with the given ID, *rnode,
   * *forknum, and *blknum are filled in (if not NULL), and returns true.
   * Otherwise returns false.
   */
  bool
  XLogRecGetBlockTag(XLogReaderState *record, uint8 block_id,
  				   RelFileNode *rnode, ForkNumber *forknum, BlockNumber *blknum)
  {
  	DecodedBkpBlock *bkpb;
  
  	if (!record->blocks[block_id].in_use)
  		return false;
  
  	bkpb = &record->blocks[block_id];
  	if (rnode)
  		*rnode = bkpb->rnode;
  	if (forknum)
  		*forknum = bkpb->forknum;
  	if (blknum)
  		*blknum = bkpb->blkno;
  	return true;
  }
  
  /*
   * Returns the data associated with a block reference, or NULL if there is
   * no data (e.g. because a full-page image was taken instead). The returned
   * pointer points to a MAXALIGNed buffer.
   */
  char *
  XLogRecGetBlockData(XLogReaderState *record, uint8 block_id, Size *len)
  {
  	DecodedBkpBlock *bkpb;
  
  	if (!record->blocks[block_id].in_use)
  		return NULL;
  
  	bkpb = &record->blocks[block_id];
  
  	if (!bkpb->has_data)
  	{
  		if (len)
  			*len = 0;
  		return NULL;
  	}
  	else
  	{
  		if (len)
  			*len = bkpb->data_len;
  		return bkpb->data;
  	}
  }
  
  /*
   * Restore a full-page image from a backup block attached to an XLOG record.
   *
   * Returns true if a full-page image is restored.
   */
  bool
  RestoreBlockImage(XLogReaderState *record, uint8 block_id, char *page)
  {
  	DecodedBkpBlock *bkpb;
  	char	   *ptr;
  	PGAlignedBlock tmp;
  
  	if (!record->blocks[block_id].in_use)
  		return false;
  	if (!record->blocks[block_id].has_image)
  		return false;
  
  	bkpb = &record->blocks[block_id];
  	ptr = bkpb->bkp_image;
  
  	if (BKPIMAGE_COMPRESSED(bkpb->bimg_info))
  	{
  		/* If a backup block image is compressed, decompress it */
  		bool		decomp_success = true;
  
  		if ((bkpb->bimg_info & BKPIMAGE_COMPRESS_PGLZ) != 0)
  		{
  			if (pglz_decompress(ptr, bkpb->bimg_len, tmp.data,
  								BLCKSZ - bkpb->hole_length, true) < 0)
  				decomp_success = false;
  		}
  		else if ((bkpb->bimg_info & BKPIMAGE_COMPRESS_LZ4) != 0)
  		{
  #ifdef USE_LZ4
  			if (LZ4_decompress_safe(ptr, tmp.data,
  									bkpb->bimg_len, BLCKSZ - bkpb->hole_length) <= 0)
  				decomp_success = false;
  #else
  			report_invalid_record(record, "image at %X/%X compressed with %s not supported by build, block %d",
  								  LSN_FORMAT_ARGS(record->ReadRecPtr),
  								  "LZ4",
  								  block_id);
  			return false;
  #endif
  		}
  		else if ((bkpb->bimg_info & BKPIMAGE_COMPRESS_ZSTD) != 0)
  		{
  #ifdef USE_ZSTD
  			size_t		decomp_result = ZSTD_decompress(tmp.data,
  														BLCKSZ - bkpb->hole_length,
  														ptr, bkpb->bimg_len);
  
  			if (ZSTD_isError(decomp_result))
  				decomp_success = false;
  #else
  			report_invalid_record(record, "image at %X/%X compressed with %s not supported by build, block %d",
  								  LSN_FORMAT_ARGS(record->ReadRecPtr),
  								  "zstd",
  								  block_id);
  			return false;
  #endif
  		}
  		else
  		{
  			report_invalid_record(record, "image at %X/%X compressed with unknown method, block %d",
  								  LSN_FORMAT_ARGS(record->ReadRecPtr),
  								  block_id);
  			return false;
  		}
  
  		if (!decomp_success)
  		{
  			report_invalid_record(record, "invalid compressed image at %X/%X, block %d",
  								  LSN_FORMAT_ARGS(record->ReadRecPtr),
  								  block_id);
  			return false;
  		}
  
  		ptr = tmp.data;
  	}
  
  	/* generate page, taking into account hole if necessary */
  	if (bkpb->hole_length == 0)
  	{
  		memcpy(page, ptr, BLCKSZ);
  	}
  	else
  	{
  		memcpy(page, ptr, bkpb->hole_offset);
  		/* must zero-fill the hole */
  		MemSet(page + bkpb->hole_offset, 0, bkpb->hole_length);
  		memcpy(page + (bkpb->hole_offset + bkpb->hole_length),
  			   ptr + bkpb->hole_offset,
  			   BLCKSZ - (bkpb->hole_offset + bkpb->hole_length));
  	}
  
  	return true;
  }
  
  #ifndef FRONTEND
  
  /*
   * Extract the FullTransactionId from a WAL record.
   */
  FullTransactionId
  XLogRecGetFullXid(XLogReaderState *record)
  {
  	TransactionId xid,
  				next_xid;
  	uint32		epoch;
  
  	/*
  	 * This function is only safe during replay, because it depends on the
  	 * replay state.  See AdvanceNextFullTransactionIdPastXid() for more.
  	 */
  	Assert(AmStartupProcess() || !IsUnderPostmaster);
  
  	xid = XLogRecGetXid(record);
  	next_xid = XidFromFullTransactionId(ShmemVariableCache->nextXid);
  	epoch = EpochFromFullTransactionId(ShmemVariableCache->nextXid);
  
  	/*
  	 * If xid is numerically greater than next_xid, it has to be from the last
  	 * epoch.
  	 */
  	if (unlikely(xid > next_xid))
  		--epoch;
  
  	return FullTransactionIdFromEpochAndXid(epoch, xid);
  }
  
  #endif
  